In general, it has been found that cancericidal drugs, such as chemotherapeutics, are also toxic to cells of normal tissues. Consequently, the side effects of such drugs can be almost as devastating to the patient as the malignant disease itself. The advent of monoclonal antibodies and peptide ligands provided a new means for improving drug specificity/selectivity. By conjugating, e.g., a cytotoxic agent to an antibody or peptide ligand directed against antigens present on malignant cells, but not present on normal cells, selective killing of malignant cells has been achieved. Many different immunoconjugates comprising an antibody attached to a cytotoxic agent have been created directed against a variety of cell-surface antigens.
Cytotoxic agents used in such immunoconjugates include radioisotopes, various plant and bacterial toxins (e.g. Pseudomonas exotoxin, diphtheria toxin, ricin, abrin, etc.), various growth factors, and more recently, agents such as caspases. Although there have been some successes, notably in lymphoma and leukemias, antibody-based therapy whether using antibodies alone or immunoconjugates, has generally not fulfilled the expected potential.
Although significant advances have been made in the treatment of malignant disease, curative regimens for most patients have not yet been developed or are associated with toxicities unattractive for the patient. Therefore, new strategies for the treatment of most malignant diseases are needed. These strategies should have as their goal, the maximization of therapeutic effect, coupled with the minimization of toxicity. One approach has involved the use of ligands specific for cell surface receptors or antibodies specific for malignant cell associated antigens as a means of targeting drugs or radioisotopes to the malignant cells. The approach is attractive for many malignant diseases because the malignant cells display a variety of tumor-restricted or upregulated antigens and/or receptors on their cell surfaces which would be available for targeting (reference). Thus far, antibody/antigen systems have been found to be better than ligand receptor systems because they are more restricted than receptors and in greater abundance on the malignant cell.
Despite these advantages, antibodies have not fulfilled their potential for many reasons. Among the reasons, antibodies are macromolecules (large molecules) that often do not effectively access and penetrate the malignant tumor. In addition, antibodies are often large immunogenic molecules and can induce an immune response in the patient directed against the therapeutic agent. In addition, antibodies often do not show sufficient specificity for the target (e.g. cancer) tissue and thus are useful in only limited therapeutic regimen.